Fix intention syncing CAS index

[andrewstucki]

Changes proposed in this PR:

Our CAS operation for updating intentions was broken because it was using the wrong config entry index. The result is that if multiple gateways bound to a single service then intentions for the second gateway weren't being properly created. This changes the sync code to use the proper index.

How I've tested this PR:

Manually and updated tests.

Checklist:

• Tests added
• CHANGELOG entry added

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[nathancoleman]

Tested by setting a breakpoint on the modified assertion in k8s_e2e_test.go. Without this change, only one intention is created, and the second fails. With this change, both intentions are successfully created.